In Tot 1.0.3 and later for macOS, a theme editor was added:
 

• Use Option key in Tot menu or Cmd-Option-Comma to open Theme Preferences
• Select the dot you want to change in the main window
• Changes to the color well are displayed immediately
• New colors are synced to all devices, including iOS
• Share your theme as .tot file with save/load in Theme Preferences
• Theme file is JSON with hex colors: easy to modify in text editor, too

NOTE: Whatever system theme (light or dark) you have macOS currently set to when you edit Tot's custom colors are what will sync and appear on iOS in the corresponding theme there. So if you have macOS set to Dark Mode and you edit the color of Dot 2, you will only see that custom color when looking at Dot 2 in Dark Mode on iOS. You'll need to update the colors for both light and dark on macOS if you want them to appear the same in both modes.